Level shifters are used in applications where there is a need to interface between different voltage domains. Level shifters can be full-swing or floating, which can be distinguished by whether the voltage domains share a common ground potential. Floating level shifters are used to shift the potential of control signals from circuits powered by low voltage power rails to the potential of circuits with floating power and ground rails. Floating level shifters are often used in gate drivers to drive power output stages in applications such as DC-DC converters, biomedical transducer drivers, Class-D audio amplifiers, MEMS, LCD drivers, high voltage charge pumps, switched-capacitor supplies, etc.
While the gate driver typically is exposed to relatively low voltages, the level shifter and possibly other components such as a bootstrap switch or diode must use high-voltage components, i.e., components which can withstand the whole input voltage range. If the high voltage range is higher than the maximum allowed voltage for the single component but still within the technology capability, cascoding approaches can also be used.
There are several ways to implement a high voltage level shifter such as cascading a high voltage device, transformer based level shifting and capacitance based level shifting. The cascoding approach tends to be slower due to the stacking of devices which typically are high voltage and therefore consume layout area, degrade (speed) performance and require accurate parasitic modelling and extraction. For high speed gate driver applications such as full-bridge, half-bridge, non-isolated buck topologies, etc. where high common mode rejection is crucial to ensure a reliable output signal, the main advantage of transformer and capacitive based level shifting solutions is that they are dynamically driven. Since inductors and capacitors block any DC component of the signal, the control signal is transformed into a sequence of pulses, either single-ended or differential, which drive the transformer or capacitor. A static control input signal is therefore translated into a continuous sequence of pulses which refresh the stage of the gate driver in order to prevent false triggering due to noise. The repetition rate is defined by application conditions like switching frequency and duty cycle and may translate in sub-nanosecond requirements. This in turn implies high current consumption.
Thus, there is a need for an improved level shifter design.